Organic light-emitting diodes (OLEDs) are of broad interest for applications in full-color display panels, flexible displays, and solid-state lighting. Much progress has been made in developing phosphorescent OLEDs (PhOLEDs) in which nearly 100% internal quantum efficiency has been achieved. However, blue emitting PhOLEDs remain challenging because high-energy triplet excitons tend to flow out without radiative decay in the emissive layer (EML). Insertion of a wide-energy-gap electron-transport material between the cathode and the blue-phosphorescent EML represents a successful strategy for confining excitons to the EML and for blocking holes, facilitating a good charge balance in the EML. Current high-performance blue PhOLEDs have been achieved by vacuum deposition of small molecules to fabricate multilayered device structures. Although it is economically highly desired to produce high-performance PhOLEDs by solution-based fabrication processes, new small-molecule or polymer-based electron transport/hole blocking materials and novel solution-processing strategies are essential to realize this goal.
Solution-based device fabrication methods, such as spray-on or spin-on deposition, ink jet printing, screen printing, and roll-to-roll printing processes, are considered critical to next generation, low cost, large area, high performance light-emitting devices. In contrast to the considerable progress in developing highly efficient PhOLEDs using vacuum deposition, reports on solution-processed devices are still relatively few. Surprisingly, nearly all prior reports on solution-processable PhOLEDs are multilayered structures that included a vacuum-deposited electron-transport layer (ETL)/hole-blocking layer (HBL). High-performance polymer-based PhOLEDs without a vacuum-deposited ETL/HBL also include a vacuum-deposited thin layer of low work function metals (e.g., Ba, Ca) or interfacial materials (e.g., LiF, CsF) inserted between the EML and cathode metals such as Al or Ag.
The longstanding challenge in solution-based fabrication of high performance PhOLEDs and other organic electronic devices is achieving orthogonal sequential solution deposition of multilayered structures. This requirement that the solvent used to deposit the overlayer thin film not dissolve or swell the underlying layer can conflict with the factors essential to good surface wetting properties of the second solution on top of the underlying layer. Others have exploited polyfluorene-based polyelectrolytes as electron transport layers in multilayered OLEDs. However, the ionic groups in such polyelectrolytes can result in undesired electrochemical doping effects and reduce the air-stability of high work function electrodes such as Al. Thus, there remains a need to develop new materials and novel processing strategies that can enable the achievement of solution-processed high-performance PhOLEDs and multilayered electronic devices in general.